The demonstration of pulmonary surfactant by electron microscopy.

IT IS NOW WIDELY ACCEPTED that the moist respiratory surface of the mammalian lung is lined by a material which is responsible for lowering the surface tension at the air-fluid interface. This material, which is generally referred to as "pulmonary surfactant", appears to account for much of the mechanical stability of the terminal air spaces during the movements of respiration (Clements et al, 1958; Pattle, 1958). Research into the biophysical and biochemical properties of pulmonary surfactant has been spurred on by the discovery that surface tension is abnormally high in the lungs of infants dying from respiratory distress syndrome (Avery and Mead, 1959). The morphological demonstration of pulmonary surfactant in situ has proven difficult. The widely used practice of preserving lung by introducing a solution of fixative through the bronchial tree causes the surfactant lining layer to be displaced. Displacement of surfactant also occurs when attempts are made to excise small blocks of tissue from the surface of unfixed lungs. Vascular perfusion of fixative, a technique often used in the preparation of organs for electron microscopy, is seldom successful when applied to lung tissue: the pulmonary vascular bed can only be perfused with fixative solutions at pressures far above the physiological range and as a result gross pulmonary oedema invariably develops. Furthermore, the surfactant lining is rapidly dissolved by the organic solvents which are used in the preparation of tissues for electron microscopy. Indeed, the results of a recent study (Meban, 1973) have suggested that chemical extraction has probably been the major cause of failure in previous attempts to visualize surfactant. In this paper an account is given of a study in which pulmonary surfactant was successfully demonstrated in situ by electron microscopy using a modified technique for tissue preparation. The lung of the Syrian hamster was used as an experimental model: in this species the micro-structure of the lung closely resembles that of Man (Ryan et al, 1969) and, in addition, tissue samples can be obtained from adult and foetal animals in a fresh state-a technical necessity for electron microscopy.